1. Field of the Invention
The present invention relates to an optical filter which absorbs a part of the optical signals passing through an optical path and restricts the intensity of transmitted light.
2. Description of the Related Art
Generally, the optimization of the configuration of a multilayer film constituting an optical filter can restrict the intensity of transmitted light by absorbing a part of the optical signals and thus an optical filter having a predetermined transmittance is obtainable. FIG. 6A shows the configuration of a conventional multilayered optical filter, and FIG. 6B shows the thickness and the refractive index of each layer in the configuration shown in FIG. 6A. The optical filter includes TiO.sub.2 layers having a relatively high refractive index (n.apprxeq.2.5) and SiO.sub.2 layers having a relatively low refractive index (n.apprxeq.1.45), in a thickness of .lambda./5 to .lambda./10, alternately deposited several times between substrates 1a and 1b, and also includes a metal film 2 (aluminum) having a lower refractive index (n.apprxeq.0.95) and a thickness of several to several tens of nanometers deposited between two adjacent SiO.sub.2 layers, for the purpose of achieving a transmittance of 30% to 40% for a central wavelength .lambda.=780 nm passing through a typical optical path.
FIG. 7 shows the reflectance R of the optical filter shown in FIG. 6 at a wavelength .lambda. and FIG. 8 shows the transmittance T of the same. As shown in FIG. 7, although the reflectance R reaches a minimum of approximately 15% when the wavelength .lambda. is nearly equal to 730 nm, it increases at a given wavelength .lambda.=780 nm. Also, as shown in FIG. 8, although the transmittance T reaches a maximum of approximately 35% when the wavelength .lambda. is nearly equal to 720 nm, it decreases slightly at a given wavelength .lambda.=780 nm.
In such an optical filter, when the reflectance increases, the reflected light adversely affects signal light. As mentioned above, the conventional optical filter has a minimum reflectance R of approximately 15% and cannot achieve a lower reflectance, for example, 3% or less. Also, as mentioned above, the conventional optical filter requires depositing, several tens of times, dielectric thin films having a high refractive index and dielectric thin films having a low refractive index, and therefore the fabricating process becomes complicated, resulting in a high fabrication cost.